Low Light Availability Research Articles

A retrospective cohort study on the influence of UV index and race/ethnicity on risk of stress and lower limb fractures

BackgroundLow vitamin D status increases the risk of stress fractures. As ultraviolet (UV) light is required for vitamin D synthesis, low UV light availability is thought to increase the risk of vitamin D insufficiency and poor bone health. The purpose of this investigation was to determine if individuals with low UV intensity at their home of record (HOR) or those with darker complexions are at increased risk of developing stress fractures and lower limb fractures during U.S. Army Basic Combat Training (BCT).MethodsThis was a retrospective cohort study using the Armed Forces Health Surveillance Center data repository. All Basic trainees were identified from January 1997 to January 2007. Cases were recruits diagnosed with stress fractures and lower limb fractures during BCT. The recruit’s home of record (HOR) was identified from the Defense Manpower Data Center database. The average annual UV intensity at the recruits’ HOR was determined using a U.S National Weather Service database and recruits were stratified into low (≤3.9); moderate (4.0-5.4), and high (≥5.5) UV index regions. Race was determined from self-reports.ResultsThe dataset had 421,461 men and 90,141women. Compared to men, women had greater risk of developing stress fractures (odds ratio (OR) = 4.5, 95% confidence interval (95%CI) = 4.4-4.7, p < 0.01). Contrary to the hypothesized effect, male and female recruits from low UV index areas had a slightly lower risk of stress fractures (male OR (low UV/high UV) = 0.92, 95%CI = 0.87-0.97; females OR = 0.89, 95%CI = 0.84-0.95, p < 0.01) and were at similar risk for lower limb fractures (male OR = 0.98, 95%CI = 0.89-1.07; female OR = 0.93, 95%CI = 0.80-1.09) than recruits from high UV index areas. Blacks had lower risk of stress and lower limb fractures than non-blacks, and there was no indication that Blacks from low UV areas were at increased risk for bone injuries.ConclusionsThe UV index at home of record is not associated with stress or lower limb fractures in BCT. These data suggest that UV intensity is not a risk factor for poor bone health in younger American adults.

Factors associated with the presence of flowering individuals of Arethusa bulbosa (Orchidaceae) in peatlands of southern Quebec

Arethusa bulbosa is a northeastern North American orchid species rare in most of its range. This study sought to determine the factors associated with the presence of Arethusa within peatlands and its absence from a priori suitable sites. Twenty-four plots with Arethusa were compared with paired plots from which the species was absent and with 22 plots in peatland where the species has never been present. Vegetation and abiotic variables were recorded within the plots, and anthropogenic variables were identified using aerial photographs. The IndVal method was used to determine whether some species were characteristic of Arethusa habitat, and discriminant function analyses were performed to determine which factors were related to occupied and unoccupied sites. No characteristic species were identified when comparing paired plots. Occupied sites were mainly characterized by lower light availability at ground level (46%; mean), higher Floristic Quality Assessment Index (35), and smaller shrubs (55 cm) than unoccupied paired plots (61%, 32, and 61 cm respectively) from the same peatland. Water pH was the main factor discriminating occupied sites (5.5) from the unoccupied sites in peatlands where the species has never been present (3.9), pH being usually higher in plots with Arethusa. Ten species were characteristic of plots with Arethusa (including Larix laricina, Sphagnum palustre, and Aulacomnium warnstorfii) when compared to a priori suitable sites. Overall, our results suggest that Arethusa settles in poor to moderate Sphagnum-dominated fen habitats rather than typical open bog habitats, as usually assumed.

Light availability determines susceptibility of reef building corals to ocean acidification

Elevated seawater pCO2, and in turn ocean acidification (OA), is now widely acknowledged to reduce calcification and growth of reef building corals. As with other environmental factors (e.g., temperature and nutrients), light availability fundamentally regulates calcification and is predicted to change for future reef environments alongside elevated pCO2 via altered physical processes (e.g., sea level rise and turbidity); however, any potential role of light in regulating the OA-induced reduction of calcification is still unknown. We employed a multifactorial growth experiment to determine how light intensity and pCO2 together modify calcification for model coral species from two key genera, Acropora horrida and Porites cylindrica, occupying similar ecological niches but with different physiologies. We show that elevated pCO2 (OA)-induced losses of calcification in the light (G L) but not darkness (G D) were greatest under low-light growth conditions, in particular for A. horrida. High-light growth conditions therefore dampened the impact of OA upon G L but not G D. Gross photosynthesis (P G) responded in a reciprocal manner to G L suggesting OA-relieved pCO2 limitation of P G under high-light growth conditions to effectively enhance G L. A multivariate analysis of past OA experiments was used to evaluate whether our test species responses were more widely applicable across their respective genera. Indeed, the light intensity for growth was identified as a significant factor influencing the OA-induced decline of calcification for species of Acropora but not Porites. Whereas low-light conditions can provide a refuge for hard corals from thermal and light stress, our study suggests that lower light availability will potentially increase the susceptibility of key coral species to OA.

Species Richness of Epiphytic Lichens in Coniferous Forests: the Effect of Canopy Openness

Epiphytic lichen communities were studied in old coniferous forests across Estonia to find out the effect of site openness on the lichen species richness. All lichen species were recorded on the basal 2 m of 105 Picea abies and 105 Pinus sylvetsris trunks. The canopy openness readings were taken with a spherical densiometer around every sampled tree. The number of species on the trunks of both tree species increased significantly with increasing canopy openness. Nine lichen species, e.g. Cladonia cenotea, Lecidea nylanderi, Platismatia glauca, were significantly favoured by higher light availability, and only one species, Coenogonium pineti, by lower light availability. The results demonstrate that in the coniferous forests higher light availability supports higher species richness of epiphytic lichens on the lower trunk of trees.

Effects of cyanobacterial summer bloom on the phytoplankton structure in an urban shallow lake, Guaíba Lake, southern Brazil

The present study aimed at evaluating the influence of cyanobacterial summer blooms on the phytoplankton structure in a subtropical urban shallow lake used as a water supply (Guaiba Lake, southern Brazil). Samples were taken close to the water intake site at Belem Novo district, which is operated and processed for public distribution by the Porto Alegre Department of Water and Sewage (DMAE). Monthly samplings were collected from October 2010 to March 2011. The following abiotic parameters were considered: N and P, conductivity, pH and water transparency, besides thermal profile, dissolved oxygen and relative water column stability (subsurface, 1.20 m and 0.5 m from the bottom). Samples for phytoplankton were collected in the water subsurface. Strong influence of high concentrations of total phosphorus and water stability was verified in the establishment of Planktothricoides raciborskii (R² = 97%, p = 0.01). Decrease in diversity, evenness and species richness evidenced the replacement of phytoplankton species by cyanobacteria biomass, especially due to the low light availability and soluble reactive phosphorus limitation. Changes in rainfall patterns related to the La Nina phenomenon might have influenced the abiotic scenario, recruiting inocula deposited on nutritionally enriched layers, favoring bloom-forming species. Additional studies are needed in order to fully understand the mechanisms involved in the complex dynamics of ecological factors related to the success of this group of organisms, especially in ecosystems used as water supply, presenting recurrent episodes of cyanobacterial blooms, as is the case of the Guaiba Lake. Key words: cyanobacteria, nutrients, La Nina, diversity.

Intensification of domestic ungulate grazing delays secondary forest succession: evidence from exclosure plots

AbstractQuestionsWhat is the relative importance of direct herbivory compared to microsite modification in ungulate impacts on secondary forest succession? Do domestic ungulate impacts differ between small‐seeded pioneers and large‐seeded late successional species?LocationBirch–beech secondary forest (Betula celtiberica and Fagus sylvatica), Bizkaia, Northern Spain.MethodsWe conducted a detailed spatial analysis of 216 permanent 1‐m2 subplots that were distributed among six plots, of which three were fenced (each plot was 3600‐m2) and three were unfenced (each plot was 1296‐m2). Within each subplot, the emergence and survival rates of all tree, shrub and vine species were monitored. In total, 21 censuses of the subplots were conducted across 4 yr (1998–2001). Ungulate abundance was measured by using the pellet counting method. Structural equation models were applied to model the entire recruitment process.ResultsEmergence and survival rates differed between small‐ and large‐seeded species, and the effect of domestic ungulates (primarily sheep) on these rates depended on seed traits. Sheep did not affect the emergence of large‐seeded species (e.g. beech and ivy), but did result in a decline in their survival. In contrast, the emergence of small‐seeded species (e.g. birch) was promoted through an increase in bare soil cover because of trampling by the ungulates, but no significant effect on survival was observed. Interestingly, the sheep prompted differences in the role of the understorey vegetation: the understorey composition was able to promote the survival of large‐seeded species growing in unfenced conditions by reducing the risk of herbivore predation, but had a negative effect on the survival of small‐seeded species because of low light availability under the shaded shrub canopies.ConclusionsOur results clearly highlight that sheep can change the seedling bank structure in a secondary temperate forest, thereby affecting the dynamics and structure of forest remnants. This impact should be included as a critical driver in current predictive models of forest dynamics in temperate regions, since herbivore pressure is increasing in forests across Europe due to an increase in wild ungulate populations and livestock.

Low light availability affects leaf gas exchange, growth and survival of Euterpe edulis seedlings transplanted into the understory of an anthropic tropical rainforest

Euterpe edulis Mart. (Arecaceae) is a threatened palm tree of the Brazilian Atlantic Rainforest understory with fundamental importance for the restoration of degraded forest environments. We assessed the leaf gas exchange, growth and survival of E. edulis seedlings transplanted at three different forest sites (S1, S2 and S3) in the same area in which cocoa trees had been cultivated in a rustic agroforestry system. Measurement was carried out during the first year after seedling transplantation. The sites were characterised according to canopy openness (CO) and total daily photosynthetic photon flux density (PPFD). Average CO and PPFD values were 13.3%, 8.0% and 6.7%, and 3.34, 2.79 and 0.62 mol m−2 d−1 for S1, S2 and S3, respectively. A progressive decline in seedling survival was observed in all sites throughout the experiment. At 387 d after planting, survival at S1, S2 and S3 was 57%, 44% and 37%, respectively. The gross light-saturated photosynthetic rate (Amax), leaf area and plant biomass were significantly higher (P < 0.05) in S1 and S2 when compared with S3. The values of dark respiration rate (R d) and photosynthetic compensation irradiance (I c) were sufficiently low for a positive carbon balance. Notwithstanding, the interpretation of results of microclimate variables together with leaf gas exchange and growth variables indicated that seedlings at all sites were in a suboptimal condition to achieve Amax, which is probably the main cause of the dramatic decline in the seedlings’ survival throughout the first year after transplantation. From a practical point of view, if the values of CO and PFD are lower than 10% and 3 mol m−2 d−1, respectively, it is suggested that the transplanting of E. edulis seedlings to the understory of abandoned agroforestry systems be accompanied by cultural practices, such as the thinning and pruning of tree tops.

Variability of phytoplankton light absorption in Canadian Arctic seas

Phytoplankton light absorption spectra (aϕ(λ)) were measured in the Canadian Arctic (i.e., the Amundsen Gulf, Canadian Arctic Archipelago, northern Baffin Bay and the Hudson Bay system) to improve algorithms used in remote‐sensing models of primary production. The absorption by algae, dominated by picophytoplankton (&lt;5 μm), was not the major light absorption factor in the four provinces; the colored dissolved organic matter (CDOM) contributed up to 70% of total light absorption. During the fall, the low total chlorophyll a‐specific aϕ*(443) (aϕ(443)/TChl a) coefficients of the Canadian High Arctic were associated with photoacclimation processes (i.e., the package effect) occurring in light‐limited environments. Low light availability and high proportion of CDOM (absorbing strongly the ultraviolet) seem to allow the growth of phytoplankton with accessory pigments absorbing light at longer wavelengths. The ratio of photoprotective and photosynthetic carotenoids (PPC:PSC) was inversely proportional with the salinity and the cell size, and mostly decreases throughout the Canadian High Arctic during fall. In return, the highest TChla‐specific phytoplankton light absorption coefficients at the blue peak (aϕ*(443)) were observed in the Hudson Bay system from September to October (i.e., fall) as well as in the Amundsen Gulf from May to July (i.e., spring/summer). These results will ultimately allow the accurate monitoring of phytoplankton biomass and productivity evolution that is likely to take place as a result of the fast‐changing Arctic environment.

Trait-related flowering patterns in submediterranean mountain meadows

The research aims were to identify the flowering pattern and the related functional strategies in submediterranean mountain meadows (central Italy) and understand their relationships with some environmental and community structure variables. The number of flowering shoots per species was counted and environmental data were collected in 40 plots during 2009. Analysis of the species and trait data sets highlighted a flowering pattern and an underlying functional pattern. Dominant species tend to bloom in the central phases of the growing season when no stress acts in the system and a long time is available for plant growth and seed maturation. This kind of species does not need functional strategies allowing the canopy fast pre-emption or the tolerance to drought stress. Non-dominant species have two groups of functional strategies that allow them to share the same flowering period of dominant ones by a different type of space occupation (spatial niche partitioning) or to flower before or after their flowering period (temporal niche partitioning). The functional strategies involved in the temporal niche partitioning have a dual ecological meaning, limiting competition with dominant species by fast growth and seed maturation (e.g., short stature, mobilisation of stored reserves, colonization of unexploited soil niches by clonal growth organs and light seeds) and enabling tolerance to drought stress (e.g., scleromorphic and succulent leaves, persistent green leaves, tap roots) and to the low light availability at the ground level owing to the change of grassland structure (e.g., tall size and upright growth form).

Growth and morphology in relation to temperature and light availability during the establishment of three invasive aquatic plant species

Invasive freshwater plants are currently spreading rapidly and this is likely to continue with further changes in global climate resulting in changes in physical and chemical conditions in freshwaters. We studied the effect of summer temperature (20°C, 25°C and 30°C) and light availability (25% and 50% of incident light availability) on shoot establishment in terms of growth rate, photosynthesis, and morphology of three invasive aquatic plants (Elodea canadensis, Egeria densa and Lagarosiphon major) in order to assess their interspecific competition. Light availability had an overall stronger effect on growth rate and plant morphology than temperature in the three species. Growth rate increased three-fold from low to high light, and low light reduced belowground biomass, increased stem length, and reduced branching and lateral spread. Photosynthetic rates were the only parameter for which temperature had an equal or stronger effect than light availability. The results show that E. canadensis has the most competitive establishment of the three species in both high and low temperature and light availability. E. densa is most competitive in warm water compared to colder water, whereas the opposite pattern is present for L. major which is most competitive in colder water. In conclusion, we suggest that that E. densa will dominate warmer, shallower waters, whereas L. major will dominate in colder, clear-water lakes, while E. canadensis continues its established role as a pioneer species that is quickly replaced by the two taller species after their arrival.

Successional changes in plant resistance and tolerance to herbivory

Despite considerable research on plant defenses, we know very little about how temporal changes in the environment may influence resistance and tolerance levels, or the costs and benefits of these defense strategies for long-lived plant species. We hypothesized that, in successional habitats, predictable environmental changes should favor strong plasticity in defense phenotypes and that the costs, benefits, and levels of tolerance and resistance will change with environmental context. Using a widely distributed, old-field perennial, late goldenrod (Solidago altissima), we conducted a field experiment to test these predictions. We planted goldenrod genets exhibiting varying levels of resistance and tolerance into three early-successional and three late-successional fields (approximately three and 15 years in age, respectively) and experimentally measured resistance and tolerance levels and their associated costs and selection coefficients. We found a significant effect of successional stage but no effect of genotype or stage-genotype interaction on defense levels. Genets planted in early-successional fields appeared to be more resistant and less tolerant to herbivory than those same genets planted in late-successional fields. There were significant trade-offs between resistance and tolerance in early-successional fields but not in late-successional fields. Each late-successional field exhibited a significant cost or selection gradient for resistance, but there was no general pattern of resistance costs or selection gradients specific to a successional stage class. In contrast, there was evidence of stage-specific costs of tolerance; late-successional fields exhibited significant costs of tolerance whereas early-successional fields did not. There was no evidence of direct selection for or against tolerance in either stage. Our results suggest that defense phenotypes might change in qualitative ways during succession. High resistance in early stages may be attributed to associational effects of the early-successional community, reducing the probability of damage, and despite a cost of tolerance in late stages, tolerance may be beneficial in mitigating the effects of both herbivory and environmental stresses (i.e., low light availability) that limit fitness in these fields. This study provides experimental evidence that succession can strongly influence defense phenotypes and promote temporal variability in relative resistance and tolerance levels.

Growth and physiological changes in saplings of Minquartia guianensis and Swietenia macrophylla during acclimation to full sunlight

Low light availability under a forest canopy often limits plant growth; however, sudden increase in light intensity may induce photoinhibition of photosynthesis. The aim of this study was to evaluate the ecophysiological changes that occur in potted plants of Minquartia guianensis and Swietenia macrophylla during the acclimation process to full sunlight. We used six full-sun independent acclimation periods (30, 60, 90, 120, 150, and 180 days) and a control kept in the shade. Shading was obtained by placing plants under the canopy of a small forest. The Fv/Fm ratio, net photosynthetic rate (PN), the maximum carboxylation velocity of Rubisco (Vcmax), maximum electron transport rate (Jmax), specific leaf area (SLA), and growth were assessed at the end of each of the six acclimation periods. Plant exposure to full sunlight caused a sudden decrease in the Fv/Fm ratio (photoinhibition) particularly in Minquartia. Photooxidation (necrotic patches) of the leaf tissue was observed in upper leaves of Minquartia. The higher PN values were observed in Swietenia under full sun, about 12 μmol(CO2) m−2 s−1. Vcmax25 values were higher after 90 days of acclimation, about 14 μmol(CO2) m−2 s−1 for Minquartia, and 35 μmol(CO2) m−2 s−1 for Swietenia. At the end of a 180-d acclimation period Jmax25 was 35 μmol(electron) m−2 s−1 for Minquartia and 60 μmol(electron) m−2 s−1 for Swietenia. SLA was higher in Swietenia than in Minquartia. In Minquartia, monthly rate of leaf production per plant (MRLP) was positive (0.22 leaf month−1) after four months in the open. Whereas, in Swietenia MRLP was positive (0.56 leaf month−1) after an acclimation period of two months. After six months in the open, height growth rates were 3.5 and 28 mm month−1 for Minquartia and Swietenia, respectively. The greater acclimation capacity of Swietenia was associated to an enhanced photosynthetic plasticity under full sun. In Minquartia, transition to full-sun conditions and lack of physiological adjustment resulted in severe photoinhibition and loss of leaves.

High-light acclimation in Quercus robur L. seedlings upon over-topping a shaded environment

High developmental plasticity at the seedling-level during acclimation to the light environment may be an important determinant of seedling establishment and growth in temperate broadleaf forests, especially in dense understories where spatial light availability can vary greatly. Pedunculate oak (Quercus robur L.) seedlings were raised beneath a range of artificial light environments (high light, partial high light and low light) to examine morphological and photosynthetic acclimation to vertically stratified light availability. Acclimation observed at the seedling level included changes in proportional distribution of biomass and leaf area ratio to enhance either light gathering under low light availability or reduction of moisture stress under high light availability. Seedling-level acclimation was partially driven by plasticity at the flush level, but plasticity of traits determining flush morphology, such as leaf number, area, and mass, was largely controlled during bud formation rather than during shoot development. Therefore, flush-level acclimation was restricted when shoots elongated from a shaded environment into a high light environment. In contrast, traits influencing leaf-level acclimation, such as leaf thickness, specific leaf area, and pigment concentrations appeared to be driven primarily by the prevailing light environment during leaf development. The plastic response in leaf traits to light environments during shoot development enabled immediate acclimation of photosynthetic capacity to the prevailing light environment. In conclusion, oak seedlings displayed a large phenotypical plasticity on multiple levels that maximized whole seedling performance.

Nitrate concentration in lettuce leaves depending on photosynthetic photon flux and nitrate concentration in the nutrient solution

The nitrate accumulation in plant tissues can occur due to low light availability. However, published studies have not linked nitrate accumulation to photosynthetic photon flux (PPF) measured during the growing period. This study aimed to evaluate the effect of the reduction of photosynthetic photon flux and the concentration of nitrate in the nutrient solution on agronomic characteristics and accumulation of nitrate in lettuce grown in hydroponics. The trial design was entirely randomized in a factorial scheme (4 x 2) with three repetitions, and four shading levels: 0, 30, 50, and 80% and two nitrate levels in nutrient solution: 5 and 10 mmol L-1. The dry matter production decreased directly and linearly with the reduction of light. The lowest level in nitrate solution also led to reduction in dry mass yield. The maximum accumulation of nitrate reached 966.3 mg kg-1 fresh mass, with PPF of 118 μmol/m²/s and 140 mg L-1 of N-NO3-1 (below the maximum levels recommended by the European Union), and the minimum of 200 mg kg-1 with PPF of 455 μmol/m²/s and 70 mg L-1 of N-NO3-1 in the nutrient solution. The reduction of nitrate in the nutrient solution from 140 to 70 mg L-1 led to the reduction of nitrate accumulation in shoots, but also caused a decrease in the production of phytomass of shoots, as well as reduced leaf area significantly.

What drives the distribution of the bloom-forming cyanobacteria Planktothrix agardhii and Cylindrospermopsis raciborskii?

The cyanobacteria Planktothrix agardhii and Cylindrospermopsis raciborskii are bloom-forming species common in eutrophic freshwaters. These filamentous species share certain physiological traits which imply that they might flourish under similar environmental conditions. We compared the distribution of the two species in a large database (940 samples) covering different climatic regions and the Northern and Southern hemispheres, and carried out laboratory experiments to compare their morphological and physiological responses. The environmental ranges of the two species overlapped with respect to temperature, light and total phosphorus (TP); however, they responded differently to environmental gradients; C.raciborskii biovolume changed gradually while P. agardhii shifted sharply from being highly dominated to a rare component of the phytoplankton. As expected, P.agardhii dominates the phytoplankton with high TP and low light availability conditions. Contrary to predictions, C.raciborskii succeeded in all climates and at temperatures as low as 11°C. Cylindrospermopsis raciborskii had higher phenotypic plasticity than P.agardhii in terms of pigments, individual size and growth rates. We conclude that the phenotypic plasticity of C.raciborskii could explain its ongoing expansion to temperate latitudes and suggest its future predominance under predicted climate-change scenarios.

Understorey diversity in southern boreal forests is regulated by productivity and its indirect impacts on resource availability and heterogeneity

Summary1. Understanding the relationship between species diversity and productivity is central to linking compositional and functional aspects of terrestrial ecosystems, and little is known about such issues in boreal forests. We used structural equation modelling (SEM) to test several hypotheses about direct and indirect influences of productivity, its correlate basal area, and resources on understorey vascular plant diversity on 2025 plots in 81 southern boreal forests in Minnesota, USA.2. We first examined the hypothesis that increasing basal area reduces plot‐scale species richness due to competitive exclusion from the most limiting resource, light. As expected, light pre‐emption increased with total basal area, which directly reduced understorey species richness. However, complex relations between basal area, dominant understorey species, and resource supply to the understorey can also influence understorey communities. Hence, we addressed whether plots with low light availability in the understorey were associated with low abundance of dominant understorey species and alleviation of competitive exclusion of other understorey species. SEM results showed that low light decreased total understorey cover, alleviating resource competition from this stratum and thus increasing understorey species richness. Furthermore, the cover of four dominant understorey species was positively correlated with light availability and negatively correlated with plot‐scale species richness.3. Aggregating data for the 25 plots at each stand, SEM showed that stand‐scale species richness was positively influenced by light heterogeneity, which in turn increased with annual above‐ground productivity.4. Species richness was positively influenced by litter %N, considered an index of nitrogen availability at the plot and stand scale.5. Synthesis. These results suggest that understorey species richness in boreal forests is regulated by productivity, but is primarily mediated by the indirect effects of productivity of the dominant producers on resource availability and heterogeneity.

Nutrients limit photosynthesis in seedlings of a lowland tropical forest tree species

We investigated how photosynthesis by understory seedlings of the lowland tropical tree species Alseis blackiana responded to 10 years of soil nutrient fertilization with N, P and K. We ask whether nutrients are limiting to light and CO(2) acquisition in a low light understory environment. We measured foliar nutrient concentrations of N, P and K, isotopic composition of carbon (δ(13)C) and nitrogen (δ(15)N), and light response curves of photosynthesis and chlorophyll fluorescence. Canopy openness was measured above each study seedling and included in statistical analyses to account for variation in light availability. Foliar N concentration increased by 20% with N addition. Foliar P concentration increased by 78% with P addition and decreased by 14% with N addition. Foliar K increased by 8% with K addition. Foliar δ(13)C showed no significant responses, and foliar δ(15)N decreased strongly with N addition, matching the low δ(15)N values of applied fertilizer. Canopy openness ranged from 0.01 to 6.71% with a mean of 1.76 ± 0.14 (± 1SE). Maximum photosynthetic CO(2) assimilation rate increased by 9% with N addition. Stomatal conductance increased with P addition and with P and K in combination. Chlorophyll fluorescence measurements revealed that quantum yield of photosystem II increased with K addition, maximum electron transport rate trended 9% greater with N addition (p = 0.07), and saturating photosynthetically active radiation increased with N addition. The results demonstrate that nutrient addition can enhance photosynthetic processes, even under low light availability.

Food Web Architecture and Basal Resources Interact to Determine Biomass and Stoichiometric Cascades along a Benthic Food Web

Understanding the effects of predators and resources on primary producers has been a major focus of interest in ecology. Within this context, the trophic cascade concept especially concerning the pelagic zone of lakes has been the focus of the majority of these studies. However, littoral food webs could be especially interesting because base trophic levels may be strongly regulated by consumers and prone to be light limited. In this study, the availability of nutrients and light and the presence of an omnivorous fish (Hyphessobrycon bifasciatus) were manipulated in enclosures placed in a humic coastal lagoon (Cabiúnas Lagoon, Macaé – RJ) to evaluate the individual and interactive effects of resource availability (nutrients and light) and food web configuration on the biomass and stoichiometry of periphyton and benthic grazers. Our findings suggest that light and nutrients interact to determine periphyton biomass and stoichiometry, which propagates to the consumer level. We observed a positive effect of the availability of nutrients on periphytic biomass and grazers' biomass, as well as a reduction of periphytic C∶N∶P ratios and an increase of grazers' N and P content. Low light availability constrained the propagation of nutrient effects on periphyton biomass and induced higher periphytic C∶N∶P ratios. The effects of fish presence strongly interacted with resource availability. In general, a positive effect of fish presence was observed for the total biomass of periphyton and grazer's biomass, especially with high resource availability, but the opposite was found for periphytic autotrophic biomass. Fish also had a significant effect on periphyton stoichiometry, but no effect was observed on grazers' stoichiometric ratios. In summary, we observed that the indirect effect of fish predation on periphyton biomass might be dependent on multiple resources and periphyton nutrient stoichiometric variation can affect consumers' stoichiometry.

Tree dynamics and co-existence in the montane-sub-alpine ecotone: the role of different light-induced strategies

Questions: Is light availability the main factor driving forest dynamics in Pyrenean sub-alpine forests? Do pines and firs differ in growth, mortality and morphological response to low light availability? Can differences in shade tolerance affect predictions of future biome changes in Pyrenean sub-alpine forests in the absence of thermal limitation? Location: Montane–sub-alpine ecotones of the Eastern Pyrenees (NE Spain). Methods: We evaluated morphological plasticity, survival and growth response of saplings of Scots pine, mountain pine and silver fir to light availability in a mixed forest ecotone. For each species, we selected 100 living and 50 dead saplings and measured size, crown morphology and light availability. A wood disk at root collar was then removed for every sapling, and models relating growth and mortality to light were obtained. Results: Fir had the lowest mortality rate (<0.1) for any given light condition. Pines had comparable responses to light availability, although in deep shade Scots pine risked higher mortality (0.35) than mountain pine (0.19). Pines and fir developed opposing strategies to light deprivation: fir employed a conservative strategy based on sacrificing height growth, whereas pines enhanced height growth to escape from shade, but at the expense of higher mortality risk. Scots pine showed higher plasticity than mountain pine for all architectural and morphological traits analysed, having higher adaptive capacity to a changing environment. Conclusions: Our results support the prediction of future biome changes in Pyrenean sub-alpine forests as silver fir and Scots pine may find appropriate conditions for colonizing mountain pine-dominated stands due to land-use change-related forest densification and climate warming-related temperature increases, respectively.

Surface facial electromyography, skin conductance, and self-reported emotional responses to light- and season-relevant stimuli in seasonal affective disorder.

Learned associations between depressive behavior and environmental stimuli signaling low light availability and winter season may play a role in seasonal affective disorder (SAD). The purpose of this study was to determine whether light and season environmental cues elicit emotional responses that are distinct in individuals with SAD. Twenty-four currently depressed SAD participants were compared to 24 demographically-matched controls with no depression history on emotional responses to outdoor scenes captured under two light intensity (i.e., clear, sunny vs. overcast sky) and three season (i.e., summer with green leaves, fall with autumn foliage, and winter with bare trees) conditions. Emotion measures included surface facial electromyography (EMG) activity in the corrugator supercilii and zygomaticus major muscle regions, skin conductance, and self-reported mood state on the Profile of Mood States Depression-Dejection Subscale. Light intensity was a more salient cue than season in determining emotional reactions among SAD participants. Relative to controls, SAD participants displayed more corrugator activity, more frequent significant skin conductance responses (SCR), greater SCR magnitude, and more self-reported depressed mood in response to overcast stimuli and less corrugator activity, lower SCR magnitude, and less self-reported depressed mood in response to sunny stimuli. Study limitations include the single, as opposed to repeated, assessment and the lack of a nonseasonal depression group. These findings suggest that extreme emotional reactivity to light-relevant stimuli may be a correlate of winter depression; and future work should examine its potential onset or maintenance significance.